1. Field of the Invention
The present invention relates in general to a hybrid drive system for driving a motor vehicle, which has an engine and a motor/generator as drive power sources, and more particularly to a hybrid drive system controlled to be operated in a suitable operation mode depending upon a specific running condition of the vehicle.
2. Discussion of the Related Art
As a drive system for driving a motor vehicle with reduced exhaust emissions, for example, there has been proposed a hybrid drive system for a motor vehicle, which includes (a) an engine operated by combustion of a fuel, (b) an electric energy storage device for storing an electric energy, (c) a motor/generator connected to the electric energy storage device, (d) a synthesizing/distributing mechanism which includes a first rotary element connected to the engine, a second rotary element connected to the motor/generator and a third rotary element connected to an output member and which is adapted to mechanically synthesize and distribute a force among the first, second and third rotary elements, and (e) a clutch which connects two elements of the first, second and third rotary elements, for rotation of the two elements as a unit. An example of such a hybrid drive system is disclosed in U.S. Pat. No. 5,258,651, wherein a planetary gear device is employed as a synthesizing/distributing mechanism.
In such a known hybrid drive system, the engine and the motor/generator are both integrally connected to rotary elements of the planetary gear device, so that the engine and the motor/generator are always rotated while the vehicle is running. When the vehicle is run with only the motor/generator being used as a drive power source, or with the electric energy storage device being charged by regenerative braking, the energy efficiency of the hybrid drive system is relatively low due to a drag resistance of the engine. Thus, the vehicle cannot necessarily be run in a suitable mode or fashion.